The present invention relates to a tunable upconverter mixer for radio communication applications.
With reference to FIG. 1, in a typical superheterodyne radio transceiver, image rejection is required. In the radio transmitter, a baseband signal or intermediate frequency signal is upconverted to a higher RF frequency for transmission. The upconversion is typically realized with a mixer, and in some radio architectures it is followed by an external filter (not shown) to remove the undesired image. For example, when a signal at an intermediate frequency fIF (derived from the baseband signal to be transmitted) is to be upconverted to a higher frequency, the analog multiplier (also known as a mixer) 10 and an oscillator 12 with frequency fLO are employed for frequency upconversion. The upper sideband frequency fLO+fIF and the lower sideband frequency fLO−fIF, are generated as a result of the mixing process. One of the sideband frequencies is the desired signal fRF, while the other one is the unwanted image frequency fIM. The goal of an upconversion process is to maintain or amplify the signal level at fRF while the image signal at fIM is to be attenuated as much as possible. This suppression of the image frequency fIM is called “image rejection.” If fLO+fIF is the desired frequency, then lower sideband rejection is required. Similarly, if fLO−fIF is the desired frequency, upper sideband rejection is required.
At high radio frequencies, a low-side oscillator fLO is usually preferred because it is easier to obtain better oscillator performance than a high-side oscillator. Consequently, the desired sideband is located at fLO+fIF while the undesired image sideband is at fLO−fIF. To suppress the image, a high-pass filter having a response as shown in FIG. 2 may be used. Costly off-chip bandpass filters are commonly employed to pass the desired sideband while rejecting the undesired image signal. Also, image reject mixer topologies are used for this purpose. However, image reject mixers typically require significantly more circuitry and power consumption of a conventional mixer.
Today, radio communication devices may support multiple frequency bands. For example, the IEEE 802.11a and b standards for wireless local area network (WLAN) applications operate in different frequency bands. The IEEE 802.11a standard operates in the 5 GHz band (5.15-5.35 GHz and 5.47-5.875 GHz), while 802.11b operates in the 2.4 GHz band (2.4-2.4825 GHz). Therefore, in a radio transceiver that operates in both bands, a different image reject filter is required for each band, thus increasing the cost.
A similar situation exists in cellular telephony where, for example, multimode phones for GSM/PCS/DCS are required. Clearly to save integrated circuit area it is highly desirable to have circuits which are usable at various frequency bands without duplicative circuitry. A tunable upconverter mixer would save significant silicon area on an integrated circuit used in these applications and reduce power consumption needs, and no such solution is heretofore known.